Optical filters have been designed that attenuate sufficiently at a particular wavelength thus leaving the rest of the visible spectrum open. Since the use of filters greater than optical density-2 (OD-2) limits the performance of pilot/operator in bright sunlight conditions, it follows that the use of OD-2 filters in a low ambient light scenario would likewise limit the performance for non-instrumental surveillance at the latter conditions. Bright light in the visible region, (e.g., wavelength from about 397 micrometers or microns to about 723 micrometers or microns) and to the infrared region to the limit of perception of human eye (which is about 100 microns or 100 micrometers), emitting from a point source relates to radiation of a different nature than that which is emitting from a wide spectrum electromagnetic radiation source extending in many directions. Radiation from a point source such as radiation from a frequency agile laser is the subject matter which is covered in greater detail hereinbelow.
Laser radiation particularly in the visible region has always presented a potential hazard to the human eye. More recently this potential has been under exploitation by military organizations both foreign and domestic. The human eye focuses light in the visible region which greatly increases the intensity of light on the retina from that striking the cornea and the lens.
Certain laser beams are 10,000 times brighter so that the sun's rays, and much more hazardous. When a beam of laser radiation is absorbed by living tissue, the extent of damage caused is dependent on several things: the energy level of the radiation, the type of tissue irradiated, and the wavelength of the laser radiation, and the time of exposure to the radiation.
Intense laser-energy when absorbed by the body is converted into heat. This heat coagulates the protein in the body's tissues (in a similar manner when boiling water coagulates egg albumin) and destroys the cells.
The human eye is the most vulnerable tissue to all types of laser radiation. The tissue in the retina (that portion of the eye upon which the light or image is focused, and specifically the fovea of the retina) is particularly susceptible to damage because the lens of the eyeball concentrates and focuses the laser beam on the fovea of the retina.
The interaction of a laser beam with eyeball tissue as received by the cornea and focused by the lens on the fovea of the retina is described as follows:
1. As the laser beam impinges on the eyeball, part of the beam is prevented from entering the eye by the iris, a colored disc behind the cornea (the outwardly convex transparent membrane forming part of the anterior outer coat of the eye); the iris acts like an automatic photographic shutter and constricts when high-intensity light impinges on the eye; PA1 2. The shutter action of the iris prevents part of the light from reaching the retina; PA1 3. The part of light which reaches the retina also effects a thin, dark-brown choroid membrane containing arteries, veins, and pigment cells which surrounds the retina, and since this membrane is dark colored and can easily absorb radiation, it must therefore be protected; PA1 4. The laser beam is converged and focused on the fovea of the retina by the lens; and, PA1 5. As projected from laser-welding principles we know that the focus is the hottest point, therefore, the laser-energy density at the fovea is about 10.sup.4 to 10.sup.6 times more concentrated than that received by the cornea and the lens. Hence, it is recognized why eye damage can occur due to this magnitude of light concentration.
As concluded from the above described interactions, it is recognized why eye damage can occur due to this magnitude.
Since the laser used as a weapon against the human eye, eyes behind optics, and optics themselves is considered a threat to U.S. forces, scientists have looked hard at contermeasure techniques. Protective goggles or glasses have been the only real product of research efforts. The bulk of the work has been in the types of lenses/filters to go in this eyewear.
Filters have been designed that attenuate sufficiently at a particular wavelength thus leaving the rest of the visible spectrum open. This is a real advantage to the pilots or operators in performing their tasks. With the advent of frequency agile lasers however, it has become necessary to filter over the entire visible region. This presents a real handicap to the operators in performing their tasks particularly in a low ambient light scenario.
Designing a filter in the form of goggles which filter over the entire visible region is not a practical approach. Another name for goggles which filter the complete visible spectrum is blindfolds. Thus, the limitation to an operator's performance is recognized, and it follows that instrument flight conditions would then be required; however, the ability to locate and pinpoint a source of light would be lost. Since OD-2 goggles pass 1/100th of the light that strikes them which means that the agile laser threat could increase laser output by 100 times and thus negate the filters. Finally, the goggles allow for exposure of both eyes simultaneously. It follows that another approach is desired since complete protection of both eyes from a first laser pulse does not appear possible. Therefore, scientists had at this point in time overlooked an approach for protection from the first laser pulse but concentrated efforts towards providing protection from the second and subsequent laser pulses. The probability however, is in the operators favor of not being damaged by the first laser pulse in that he will not be looking directly at the laser source when the first pulse arrives or that the first pulse will be off target but still detectable. This scheme also protects the operator from instinctively looking towards the source when the next pulse arrives. Applicants' cross-referenced, concurrently filed application is designed with the emphasis to operator's eye protection. Protection of optics from damaging laser flashes is a desirable feature which is described below and which also prevents eye damage in location technique.
The invention described hereinbelow provides optics protection while providing a technique to pinpoint the location of a hostile laser source. A system which provides protection from laser pulses including the first one while also avoiding the risk of damage to optics is a desirable enovation. The operator will also suffer no degradation of his performance due to insufficient light required if the optics (camera) employed to pinpoint the hostile laser source location is provided protection.
An object of this invention is to provide a technique to pinpoint the location of a hostile laser source without placing the operator at risk for eye damage.
Another object of this invention is to provide a technique to pinpoint the location of a hostile laser source which offers protection to the optics employed in the technique.
Still a further object of this invention is to additionally provide a detector for a laser pulse (flash) which immediately causes the detected laser pulse to be frozen on a monitor whereby the laser source appears as a bright spot which is easily pinpointed.